Here's a screenshot http://snid.es/d156c6b7deea
In mine, under profiles/keys, I've set ⌘s to [17~
This is essentially F6, which I then bind in vim to
nnoremap <F6> :w<CR> imap <F6> <Esc>:w<CR>
I do this with a lot of ⌘ or otherwise unbindable settings (again, this only effects iterm2). This is how you can get iterm2 to pick up any keybinding and send it to vim as something else.